Atrial fibrillation is an irregular heart rhythm that adversely affects approximately 2.5 million people in the U.S. It is believed that at least one-third of all atrial fibrillation originates near the ostium of the pulmonary veins, and that the optimal treatment technique is to ablate these focal areas through the creation of circumferential or linear lesions around the ostia of the pulmonary veins.
Heretofore, the standard ablation platform has been radio-frequency energy. However, radio-frequency energy technology is not amenable to safely producing circumferential lesions without the potential for some serious complications, including stenosis and stroke. In addition, the ablation of myocardial cells with heating energy also alters the extracellular matrix proteins, causing the matrix to collapse. Also, radio-frequency energy is known to damage the lining of the heart, which may account for thromboembolic complications.
Cryoablation of myocardial tissue has a long, successful history of use in open-heart surgery. Further, the use of cryoablation does not seem to cause extracellular matrix changes or do damage to the endocardium, allowing the correct lesion size to be created for therapeutic benefit. The cooling associated with cryoablation also has the natural tendency to freeze stationary tissue, rather than flowing blood. As a consequence, clot-related complications are greatly reduced.
Cryoablation of myocardial tissue via a catheter reduces many of the complications associated with open-heart surgery. Still, there are several complications that must be overcome to efficiently deliver cryo-energy to myocardial tissue. For example, a low temperature medium such as a refrigerant must be delivered to the general location of the tissue to be cryoablated. Thus, the catheter must contain structures for delivering the refrigerant to the target area and for transferring heat from the target tissue to the refrigerant. To reach the target area, these catheter structures must be advanced through portions of a patient's vasculature, often along extremely tortuous paths. Note; for purposes of this disclosure, the term “vasculature” including derivatives thereof, is herein intended to mean any cavity or lumen within the body which is defined at least in part by a tissue wall, to specifically include the cardiac chambers, arterial vessels and the venous vessels. Thus, the entire catheter must be considerably flexible and generally must contain some mechanism to steer the catheter as the catheter navigates through the vasculature.
Another factor that must be considered when contemplating the use of a catheter to cryoablate myocardial tissue for the treatment of atrial fibrillation is the electrical conductivity of the materials used to construct the catheter. Specifically, the cryoablation catheter may include an electrode to first map cardiac electrical signals for the purpose of selecting target tissue for cryoablation. In this case, it is generally desirable that the catheter be constructed of materials that are electrical insulators to avoid the interference with the mapping electrode. On the other hand, thermally conductive materials are generally required to transfer heat from the target tissue to the refrigerant.
In light of the above it is an object of the present invention to provide a catheter for cryoablating internal tissue. It is yet another object of the present invention to provide a segment for a cardiac cryoablation catheter for transferring heat from target tissue to a refrigerant. Yet another object of the present invention is to provide a heat transfer segment for a cryoablation catheter that is flexible enough to be advanced through the vasculature of a patient and positioned adjacent preselected myocardial tissue. It is still another object of the present invention to provide a heat transfer segment for a cryoablation catheter that also functions as an articulation segment that is controllable from an extracorporeal location to steer the catheter during advancement of the catheter through the vasculature of a patient. Still another object of the present invention is to provide a heat transfer segment for a cryoablation catheter that can be selectively deflected from an extracorporeal location to reconfigure the distal end of the catheter into a selected shape near the tissue to be cryoablated. It is yet another object of the present invention to provide a heat transfer segment for a cryoablation catheter having a selective distribution of thermally conductive material to allow for the cryoablation of selectively shaped lesions to include annular shaped lesions and linear shaped lesions. Still another object of the present invention is to provide a heat transfer segment for a cryoablation catheter that does not interfere with the catheter's mapping electrode. Yet another object of the present invention is to provide a catheter and a method of use for cryoablation of tissue which is easy to use, relatively simple to manufacture, and comparatively cost effective.